Robotically controlled instruments are often used in minimally invasive surgical procedures. An existing architecture for instruments in a surgical system includes an end effector or tool such as forceps, a scalpel, scissors, a wire loop, or a cauterizing instrument mounted at the distal end of an extension, which may also be referred to herein as the main tube of the instrument. During a surgical procedure, the end effector and the distal end of the main tube can be inserted through a small incision or a natural orifice of a patient to position the end effector at a work site within the body of the patient. Tendons, which can be cables or similar structures, extend through the main tube of the instrument and connect the end effector to a transmission and actuation mechanism, which may be referred to herein as a backend mechanism. For robotic operation of the surgical instrument, the backend mechanism at the proximal end of the instrument is motor driven to pull on the tendons and thereby move or otherwise operate the end effector. A computing system may be used to provide a user interface for a surgeon or other user to control the instrument.
Certain robotically controlled surgical instruments have flexible main tubes that are able to bend as necessary to follow a natural lumen, such as a portion of the digestive tract of a patient or for insertion through a curved guide tube that provides an improved approach direction to the surgical site when compared to a straight approach. Whether inserted directly or through a guide, the main tube of a flexible surgical instrument will generally have multiple bends at locations that may vary during a surgical procedure and may vary from one procedure to another. At these bends, the tendons running through the instrument may rub against the inside wall of the main tube of the instrument and against each other, and friction generated due to these bends can increase the forces required to move the tendons to operate the end effector at the distal end of the main tube. Additionally, these frictional forces tend to be higher at zero velocity that at low (e.g., non-zero) velocities, resulting in what is commonly called “stick-slip motion” (and sometimes referred to as “stiction”) in response to changes in tendon load. This stick-slip motion makes smooth robotic control of small movements of the end effector difficult to achieve. The large friction also makes construction of small-diameter flexible surgical instruments more difficult because mechanical structures must be designed to be robust enough to withstand the large forces applied.
In many applications, it is desirable to provide actuation cables for controlling the end effector that have minimal friction to reduce the negative impacts of friction on control of the end effector. However, in capstan drives that use friction to retain a driven cable, it is desirable to provide an interface between the capstan surface and the cable that provides a relatively high friction to maintain the coupling between the cable and the capstan surface. Existing systems typically address one of the following two requirements. Using a low-friction cable reduces the negative impacts of friction on the control of the end effector, but reduces the force that can be applied by the capstan due to slippage of the low-friction cable. Alternatively, using a high-friction cable impedes the control of the end effector, but increases the force that can be applied by the capstan due to reduced slippage of the high-friction cable.
Accordingly, instruments are desired that provide a cable offering sufficiently low friction to enable control of an end effector as well as significant friction between the cable and a capstan surface to properly drive the cable.
Throughout the description, similar reference numbers may be used to identify similar elements.